Castling is the only move in chess that moves two pieces at the same time: the king and one rook. The king slides two squares toward the rook, and the rook jumps over him to land right next to him. In a single move, you tuck your king safely into a corner of the board and activate a rook that was sitting idle.

There are two versions: kingside castling (written O-O), also called short castling, and queenside castling (written O-O-O), also called long castling.

How castling works

For White’s kingside castle: the king goes from e1 to g1, and the h1-rook comes to f1. For queenside: the king goes from e1 to c1, and the a1-rook comes to d1. For Black, it’s the exact mirror on the 8th rank.

Kingside castling in the Italian Game: the white king slips to g1 and the h1-rook shields him from f1.

A simple way to remember it: the king always moves exactly two squares, never more. The rook then hops to the other side of him.

The castling rules

Castling is only allowed when all of these conditions are met:

  • The king has never moved during the game.
  • The rook involved has never moved either.
  • Every square between the king and the rook is empty.
  • The king is not in check at the moment of castling.
  • The king does not pass through any square attacked by an enemy piece, and does not land on an attacked square.

One subtlety worth knowing: only the king’s path matters. The rook may cross an attacked square without any problem (a frequent case in queenside castling with the b1 or b8 square).

Kingside or queenside?

Kingside castling is faster to prepare (two pieces to develop instead of three) and leaves the king well sheltered behind the f, g and h pawns. It’s by far the most common choice. Queenside castling puts the rook straight onto the d-file, often a very active one, but it exposes the king a little more and usually costs an extra move to tuck the king away on b1.

In most openings, castling before move 10 is an excellent habit: a king stuck in the center is the number one cause of quick defeats among beginners.

Frequently asked questions

Can you castle to get out of check?

No. If your king is in check, castling is forbidden. You must deal with the check another way first: move the king, block the attack, or capture the checking piece. It’s one of the rules beginners forget most often.

Can you castle if the rook is under attack?

Yes. The castling conditions only concern the king: he alone must not be in check, nor cross an attacked square. A rook that’s attacked, or that passes through a square controlled by the opponent, does not prevent castling.

How is castling written in notation?

Kingside castling is written O-O and queenside castling O-O-O, in every language. It’s a special notation: no starting or ending square is given, because the movement is always identical.